Fast imaging is defined herein as imaging sequences where the time to repeat (TR) is less than the relaxation time T2. Fast imaging methods in general, use tip angles of less than 90 degrees with gradient recalled echoes rather than 180 degrees RF pulse generated echoes. This sequence often results in an image artifact in the form of a high intensity band around the center of the image. The artifact is caused by the contribution of the transverse magnetization (Mxy) to the steady-state condition prior to the application of the next radio frequency pulse. That is, any residual transverse magnetization remaining at the end of an acquisition period is transformed into unwanted phase coherence by the subsequent RF pulse.
See the following articles, for example prior art systems:
(1) "Contrast in Rapid MRI:T1 and T2 Weighted Imaging" by Buxton R. B. et al, published in the Journal of Computer Assisted Tomography, vol. 11, pp7-15, (1986); PA0 (2) "Mechanisms of Contrast in NMR Imaging" by Wehrli, R. W. et al published in the Journal of Computer Assisted Tomography, vol. 8, pp369-380 (1984); PA0 (3) "Transverse Coherence in Rapid Flash NMR Imaging" by Frahm J. et al published in the Journal of Magnetic Resonance, vol. 72, pp307-314 (1987). PA0 (4) "Visualization of Cerebral and Vascular Abnormalities by NMR Imaging. The effects of Imaging Parameters on Contrast" by Crooks, L. E. et al published by Radiology, vol. 144, pp843-852 (1982); and PA0 (5) "Rapid NMR Imaging Using Low Flip Angle Pulses" published in the Journal of Magnetic Resonance, vol. 67, pp258-266 (1986).
Presently, there are no effective fast imaging systems which provide T1 contrast images and also remove the high intensity artifact. To remove the artifact, the transverse magnetization (Mxy) should be zero or close to zero prior to the application of the next RF pulse. The Mxy, unfortunately, does not naturally decay in the short time periods used by fast imaging procedures. Accordingly, it is necessary to manipulate various controls of the MRI systems to minimize or eliminate the Mxy magnetization.
One of the fast imaging methods presently used to obtain good T1 contrast images comprises the use of a "spoiler" gradient at the end of each sequence (prior to the next application of an RF pulse) and to change the spoiler gradient from cycle to cycle. The spoiler destroys the transverse (Mxy) magnetization coherence. A problem with the spoiler gradient method of eliminating transverse magnetization is that near the image center, the spoiler gradient is very small, and accordingly, near the center of the spoiler gradient, the Mxy magnetization is not eliminated. Actually the band in the image around zero is quite broad and for this reason when using the spoiler gradients, there is often a high intensity bright band around the zero in the direction of the spoiler gradient.
Another problem is the fact that the varying spoiler gradient creates varying eddy currents which further degrade longitudinal relaxation time T1 contrast.
In spin warp imaging, the incremental phase encoding gradient which varies from cycle to cycle acts to prevent the transverse magnetization from reaching a steady state. Here again at the center where the absolute value of the gradient is small the magnetization in the XY plane does tend to reach a steady state and there is an artifact along the center of the image. This signal from spins close to the center is brighter and more intense when compared to spins more distant from the center.
Presently, when using fast imaging sequences a refocusing method is used to get T2 (transverse relaxation time) contrast images. This method refocuses or cancels the defocusing effect of the phase encoding gradient. The refocusing gradients are applied between the sampling of the signal and the application of the next RF pulse by applying opposite going phase encoding pulses after signal acquisition and prior to application of the subsequent RF pulses. The opposite going phase encoded pulses refocus the coherence of the transverse magnetization Mxy, and steady state is established. This approach creates contrast which resemble those of a steady state free precession experiment. (Thus, the image generated contrast is approximately determined by T1/T2 and the overall image contrast is reduced). Accordingly, among other things, one fast imaging method ("spoiling fast scan") provides T1 contrasted images with serious artifact problems. While the other method ("refocusing fast scan") provides T1/T2 contrast but reduces the overall image contrast. Accordingly, a solution is required which provides good T1 contrast and essentially eliminates the artifact.
In this invention an elegant method of implementing the destruction of the coherence of the transverse magnetization is disclosed. The transverse magnetization is destroyed throughout all the image without creating eddy currents. Thus, this invention enables obtaining artifact-free images using fast imaging techniques with better T1 contrast than is done in the known prior art.